1. Field of the Invention
The present invention relates to semiconductor integrated circuits, and in particular to circuits in which the use of particular data in particular circuits is desired to be restricted.
2. Description of the Related Art
In many electronic devices it is often necessary to make restriction on the data that can be used within the device. This is illustrated in the following example. In a subscriber based pay-television system, television signals broadcast over air in an encrypted form are received and a set-top-box is used to decrypt the signals so that broadcast services may be viewed. In such a system, one or more cryptographic keys are required to decrypt the encrypted television signals. A set-top-box belonging to a subscriber acquires the cryptographic keys necessary to decrypt the television signals corresponding to a particular broadcast service only when the necessary payment has been made to the broadcast service provider.
Usually, there are several broadcast service providers, each providing their own set of broadcast services. Accordingly, the set of cryptographic keys used to decrypt the television signals of one broadcast service provider are different to the set of cryptographic keys used to decrypt the television signals of a different broadcast service provider. Usually, when set-top-boxes are distributed by a set-top-box manufacturer or sold by a retailer, each set-top-box is intended to be used in conjunction with the services provided by a particular broadcast service provider. Accordingly, it is desirable to prevent cryptographic keys associated with one broadcast service provider from being used in a set-top-box that is intended to be used only in conjunction with the services of another broadcast service provider.
Often, a subscription is paid to enable access to broadcast services for a limited period of time, for example to enable access to a particular film channel for a year. In this case, it is desirable to allow the cryptographic keys used to decrypt the television signals corresponding to the service to be used in a set-top-box for a year, and thereafter to prevent the cryptographic keys from being used by the set-top-box.
The set-top-boxes are controlled by software which may be updated periodically to provide improved user interfaces, to fix software bugs or to expand the range of services or general functionality provided by a set-top-box. When a software upgrade is downloaded to a set-top-box it is desirable that the old version of the software is prevented from being used by the set-top-box to ensure that only the most recent software is used. The software requires one or more cryptographic keys to function properly. Usually, when a piece of software is updated, the cryptographic keys used by the software are also updated. In this case, it is desirable to prevent cryptographic keys associated with an old version of software from being used in a set-top-box to in turn prevent the old software from being used
We have appreciated that, in general, the use of particular data of many types within a device may need to be restricted to a particular group of devices, to a particular period of time or be subjected to many other types of restriction.